Method of photoelectric copying



p 2, 1969 M. WALY 3,464,818

METHOD OF PHOTOELECTRIC COPYING Filed June 22, 1967 5 Sheets-Sheet 1Arrpeygys Sept. 2, 1969 M. WALY 3,464,818

METHOD OF PHO'I'OELECTRIC COPYING Filed June 22, 1967 3 Sheets-Sheet 2//V MFA/T02 MOI/STA PHA wALY Sept. 2, 1969 M. WALY, 3,464,818

METHOD OF raowommcmc comm Filed June 22, 1967 s Sheets-Sheet 5 UnitedStates Patent Office 3,464,818 Patented Sept. 2, 1969 Int. Cl. G03g13722; H01j 37/26 US. CI. 96-13 8 Claims ABSTRACT OF THE DISCLOSURE Themethod of the invention consists in forming an electrostatic image upona photoconductive surface, in uniformly charging the insulating surfaceof a nonphotosensitive and nonphotoconductive paper sheet, in bringingsaid photoconductive surface in the presence of said insulating surfaceat a distance thereof comprised between 0.01 and 120 microns bysubjecting said surface to an electric field comprising a high frequencyalternative current component and a direct current component and indeveloping and fixing the electrostatic image so transferred on saidinsulating surface. The frequency of the alternative current componentis comprised between 1.2 kilohertz and 3000 megahertz.

The present invention essentially relates to a method of photoelectriccopying or manifolding; more specifically it is concerned with a newxerographic process for making true copies of documents from opaque ortransparent on light and cheap papers; these papers also are a subjectmatter of the invention in as much as they have been subject to aspecial preparation to be described hereinafter.

The usual xerographic art presently amounts to two methods which arerecalled hereinafter. The first of these methods consists in exposing,through contact or an optical way, to the document to be copied, a metalplate or drum coated With a thin photoconductive layer previouslycharged with electricity. In most cases, the photoconductive layer ofamorphous selenium deposited in a vacuum on an aluminum plate or drumpreviously suitably treated. This selenium coat retains the electriccharges in the darkness and dissipates them when subject to an actinicillumination. When after charging such a surface in the darkness, anoptical image of the document to be reproduced or copied is projectedthereon, the spots, locations or areas of this surface, corresponding tothe opaque Zones of said document (black lines of a letter or image),keep their electric charge whereas the spwts, areas or locations,corresponding to the blanks of the document, loose the charge. Thus anelectrostatic image of said document is obtained. While still operatingor working in the darkness, said layer is dusted with finely dividedelectroscopic powder which adheres or sticks to the areas or spots leftcharged thereby forming an electrostatic image of said document. Whilestill in the darkness, a paper sheet, web, band or tape is brought intocontact or engagement with the plate or drum. The particles of theelectroscopic powder are transferred to the paper on which the replica,reproduction or copy has to be made with the assistance of an electriccharge applied to the back of the latter. These particles temporarilyadhere or stick by induction on said paper, and, after treatment withinfra-red radiation, a permanent, contrasted and fixed document isachieved on this paper. It should be noted that the photoconductivesurface made of amorphous selenium is a P-type conductor, whichconductor may also receive a positive charge, and this requires or callsfor the use of a negatively charged electroscopic powder.

In the second of these usual xerographic methods, the copying orreproduction of the document is directly elfected on a photoconductivesurface supported or carried by a paper sheet. In this case, thephotoconductor, generally consisting of zinc oxide, is finely dispersedin a highly insulating varnish applied in the form of a thin coat orlayer to an ordinary paper. This zinc oxide containing layer or coat isthen charged electrostatically, so as to impart thereto a negativepolarity with respect to the backing paper or paper support. This layeror coat is then exposed in a known manner (illumination of the documentand projecting of the pattern thereof by an optical system) to thedocument to be copied or reproduced. The electrostatic image achieved onthis layer or coat is then developed by means of an electroscopicsubstance applied to the photoconductive surface by well knownprocesses, either in a smoke chamber, or within an insulating liquidmedium (petroleum, etc.) containing carbon black or flame blackdispersed and maintained in suspension, either by means of a magneticbrush or even by directly applying a positively charged electroscopicpowder.

Both of these known xerographic methods suffer from significantdrawbacks:

In the first one of these methods, the apparatus is delicate and itsmaintenance which represents a substantial part of the cost price of thecopy owning to the fact that the latter is achieved on almost ordinarypaper, prohibitively increases this cost price.

A second inconvenience of this method resides in the fact that the drumcomprising the photoconductive layer or coat, has to be subject to aperiodical cleaning after the making of each reproduction or copy, so asto remove the residual electroscopic powder. In spite of such a cleaningbeing carried out with rotary brushes having a smooth or soft action,there occurs an attack of the photoconductive layer or coat thethickness of which should have to be maintained strictly constant (about20 microns); it results therefrom a distortion in the copy orreproduction.

The second one of said methods produces expensive copies, on account ofthe fact that each one of the latter comprises a photoconductive layeror coat on the one side of a backing paper or a paper support which hadto be coated with a thin layer of zinc oxide. For this purpose, thisbacking paper or paper support is at first finely crushed, ground,pounded or milled and dispersed within a highly insulating binder whichis soluble in a hydrocarbon. In order to avoid the penetration of thesolvent while spreading the photoconductive layer or coat, the paper iscarefully provided on the recto and verso with another suitable varnish,which is insoluble in said hydrocarbon. Consequently, the production onan industrial scale of such a paper, the photoconductive layer thicknessof which should be kept constant, to 2 or 3 microns for a usefulthickness of about 15 to 20 microns and the accuracy required duringcoating by means of special spreading machines are as many factors whichsubstantially increase the cost price of this paper and decrease orreduce the efficiency of the method. Moreover, the paper used is veryheavy: the combined weight of the support and photoconductive layer orcoat amounts to about g./m. whereas the weight of a paper of current usefor ofiice work is of the order of magnitude of 60 g./m.

In summary, the effect of the weight and the high cost of this paper isto exclude this second method from the techniques of copying orreproduction on an industrial scale.

The method according to the present invention allows to obviate to theaforesaid drawbacks, by the fact that it is of a low cost price inrelation of the use or working of a simplified apparatus of easymaintenance and the use of a cheap and light current paper, to achievecopies or reproductions; in addition, it yields a very true copy of theoriginal, even of the half-tints thereof; it may be adapted to thepanchromatic copying and enables ready obtainment of a plurality ofidentical copies.

This method comprises the steps of forming, from the original to becopied, an electrostatic image on a photoconductive surface carried,supported, or backed by a base conducting static electricity, uniformlycharging with static electricity an insulating surface of anonphotosensitive and nonphotoconductive paper sheet or the like, thenbringing or feeding with the exclusion of active light saidphotoconductive surface into contact or engagement with or in theimmediate vicinity of said insulating surface, by subjecting the area orzone wherein both of said surfaces are thus fed, to the action of anelectric field having a high frequency A.C. component and a continuousor DC. component, and developing and fixing in a manner known per se,after having separated said photoconductive surface from said papersheet, the electrostatic image formed on the insulating surface of thispaper sheet.

The original to be reproduced or copied may be printed, written inpencil or in ink, be in color or in black and white, and carried orbacked by an opaque, translucent or transparent support; this support orbacking may be a paper or a plastic film or any other support ofsuitable nature adapted to comprise characters or types, signs orpatterns or designs; this original may consist of a photosensitive ornonphotosensitive paper, film, skin or plate comprising a pattern ordesign (text and/or drawings), obtained by any photographic orxerographic process or any other means. This pattern or design may be apositive or a negative; it may furthermore correspond to a light imageor to an inverted or reversed right-left image.

The photoconductive surface, upon which is formed or built up the firstaforesaid electrostatic image, is chiefly made from or contains at leastone of the following compounds: zinc oxide, cadmium sulphide,anthracene, the triazoles, the oxaziles, the thiazoles, the triazines,etc.

In the case where the selected photoconductor is zinc oxide, the uniformcharge imparted to the photoconductive surface will be a negative chargeand it will then be necessary that the uniform charge imparted to theinsulating surface of the paper be negative; with other photoconductors,it is a positive charge which should be imparted to the photoconductivesurface and it will then be necessary to positively charge saidinsulating surface. In general, it matters that this insulating surfaceand the photoconductive surface have been charged with staticelectricity of constant sign.

The concept of mutual contacting or engagement of both aforesaidsurfaces has not to be defined; the concept of the immediate vicinity ofthese two surfaces means that both of said surfaces are placed at adistance from each other that the copying or reproduction takes placewith the same results as if they were in mutual contact, the conditionsor requirements of the process remaining besides unchanged; this casemay correspond to the presence of a free intermediate space or gap ofsmall dimensions or size, generally occupied by air, between bothaforesaid surfaces, then disposed preferably in confronting or opposedrelationship; it may also correspond to the juxtaposition of thesupports or backings of both surfaces or even to the juxtaposition ofone of these surfaces with the other surface; intermediate layers orcoats of various natures, not connected or bounded to said backings orsupports may also be provided between said surfaces. According to thepresent invention, the distance between both aforesaid surfaces is inthe range between 0.01 and 120 microns, this last value corresponding tothe upper limit specifying what is meant by immediate or closeproximity.

The continuous or DC. component of said-electric field is preferablyobtained from a direct current source connected to a conductive supportor backing of the said paper sheet or web; this continuous or DCcomponent may also correspond to the induction effect produced by anelectret disposed near the area wherein both aforesaid surfaces are inmutual contact or in close or immediate proximity to each other.

According to the present invention, the frequency of the alternativecurrent component should correspond to a value ranging from 1.2kilohertz up to 3000 megahertz and, preferably from 15 kilohertz to3000- megahertz; this frequency may be supplied by any device known perse, in particular by a high frequency oscillator.

The area of both aforesaid surfaces, wherein the latter are subject tothe action of the electric field with a high frequency component, may beof a more or less large extent; this area, which has in principle astationary position in space, may be or not movable with respect to bothaforesaid surfaces according as the process used is continuous or not.In the case of a continuous process, both aforesaid surfaces will movewith the same speed at this zone or area during a time long enough forthe whole pattern or design to be copied (that is the electrostaticimage to form on said insulating surface from the electrostatic image onthe photoconductive surface) to pass or move in said area or zone;preferably, in the case of such a continuous process, said area willhave a small width of the order of a few millimeters, so as to graduallyform the final electrostatic image by sweeping (in successive garallelrows) both aforesaid surfaces with the electric eld.

With the term paper or the like or more simply by the term paper usedhereinafter, is meant a paper of cellulosic nature or even a film orskin of plastic material of suitable nature or still any other suitablethin support, carrier or backing. In general, a sized or nonsizedcellulosic paper of current type will be used for economys sake of themethod; it is compulsory or requisite that such a paper has at least oneinsulating surface for the formation of the electrostatic image underthe conditions or according to the requirements of the process of theinvention; the presence of this insulating surface may be due to thefact that the paper is itself dry and insulating enough throughout itsthickness or even it may be due to an insulating layer or coat appliedon this paper by any suitable means or formed within an initialsuperficial coat or layer of this paper, for example by absorptionand/or spreading.

According to a further feature of the present invention, a paper ofcurrent type, of the kind used in offices, is used, which will have beentreated or processed or the manufacturing method of which will have beenconducted in such a manner that it comprises, on at least one of itssides, an outer insulating coat or layer, which may extend throughoutthe whole thickness of this paper; preferably, this layer or coat willconsist of a dispersion of insulating substance in a suitable binder.The paper weight such as used in the method of the invention, may be ofthe order of 40 to 70 g./m. preferably in the vicinity of 55 g./m.

The invention will be better understood and further objects, features,details and advantages thereof will become apparent as the followingdescription proceeds, with reference to the diagrammatic drawings, givenby way of example only and wherein:

FIGURE 1 is a diagrammatic of a first device for carrying out the methodof the invention in a continuous operation;

FIGURE 2 is a diagrammatic view at an enlarged scale of the dischargingzone of the device according to FIG- URE l, on which view a symbolicillustration of the distribution of charges has been adopted in order toshown the assumed mechanism of the process;

FIGURE 3 is a diagrammatic view illustrating the principle of theinvention in the case of a discontinuous process;

FIGURE 4 is a diagrammatic view showing the principle of the inventionand corresponding to an alternative embodiment or modification of thedevice of FIG- URE 3;

FIGURE 5 shows a diagrammatic view of a second device for carrying outthe method of the invention;

FIGURE 6 is a cross-sectional view of a scorotron-type device for theuniform and regular electrostatic charging of a surface;

FIGURE 7 is a diagrammatic view of a third device for carrying out themethod according to the invention;

FIGURE 8 is a diagrammatic view of a fourth device for carrying out themethod according to the invention.

The device of FIGURE 1 comprises two metal drums 3 and 4 withsubstantially parallel axes, which may drive or carry along, in thedirection shown by the arrows f, a paper sheet or web 2 having aphotoconductive surface and called hereinafter photoconductive paper aswell as a light paper sheet or web 1 with an insulating surface; thesedrums have like diameters and rotate at the same speed; they are mountedon a common frame 15 made of insulating material; the Spacing of thedrums 3 and 4 is such that the distance between the photoconductivesurface of the photoconductive paper 2 and the insulating surface ofpaper 1, which surfaces are disposed in confronting or opposed relationto each other, be comprised between 0.01 and 120 microns.

The axes 16 and 17, respectively, of the drums 3 and 4 are connected,through the medium of frictional or sliding contacts 5 and 6,respectively, with an electronic device which comprises a high frequencycircuit and a direct current circuit. The drum 3 is grounded through aresistor 12 mounted in parallel or as a shunt with respect to or acrossor as a branching off the capacitor 7 and the drum 14 is connectedthrough the capacitor 18 to the high frequency oscillator 9 of the highfrequency circuit as well as through the resistor 11 to a source ofdirect current of negative polarity (negatively or positively boosted,that is stepped down or stepped up, and rectified current of the mains).The cold point of the oscillator 9 as well as the positive pole of thedirect current source ltl are grounded.

Between the common point of the high frequency circuit and of the directcurrent circuit and: the ground, a small neon glow lamp 8 is inserted inseries with a resistor 13 across which is connected or branched off acapacitor in parallel relation thereto; this arrangement or assemblyacts as a stabilizer for the voltage at the axis 17.

The high frequency path is set up as follows: oscillator 9, capacitor18; frictional or sliding contact 6, axis 17, mass of the drum 4,capacitor 14, mass of the drum 3, axis 16, frictional or sliding contact5, capacitor 7, ground. The capacitor 14 consists of the opposed orconfronting portions of both drums 3 and 4 as well as by the areas ofthe paper sheets or webs disposed on these portions and by the air lyingbetween the outer surfaces of these areas.

The path followed by the direct current produced by the source 10comprises the resistor 11, the frictional or sliding contact 6, theaxis, shaft or spindle 17, the mass of the drum 4, the area 14, the massof the drum 3, the frictional or sliding contact 5, the axis, shaft orspindle 16, the resistor 12. and the ground.

The device of FIGURE 1 operates in the following manner: thephotoconductive coat or layer of the sheet or web 2 being formed by thezinc oxide dispersed within an insulating varnish, said layer or coathas been negatively charged at the surface thereof in the darkness by acorona-discharge; then the pattern or design of the document to becopied or reproduced has been optically projected, so as to form anegative electrostatic image, the spots or areas of the photoconductivesurface corresponding to the portions in black of the original, havingbeen discharged during the projection of said pattern or design. Thepaper sheet or web 1 has been uniformly charged with static electricityof negative sign. The sheets or webs 1 and 2 are then secured orfastened upon the drums 4 and 3, respectively; the high frequencyoscillator 9 and the source of direct current 10 are then switched on;rotation is imparted to the drums 3 and 4, for example in the directionof the arrows and the peripheral speed of the drums is adjusted to avalue lying between 2 cm./ s. and cm./s. When both sheets or webs havetravelled or moved in confronting, registering or opposed relationshipover their whole length, both drums are stopped and the sheet or web 1is removed from the drum 4.

The development of this sheet 1 is then effected by the methods known inelectrophotography. For example, this sheet or web is introduced into adust chamber, where it is subject to a cascade or succession ofpositively charged electroscopic particles or the surface of this paperis sprinkled or sprayed with 'an insulating liquid containing insuspension positively charged flame or lamp black particles; accordingto this last method, the development occurs through process which is acombination of electrophoresis and dielectrophoresis.

The image is then fixed by methods known per se by subjecting the papersheet or web, upon which :adhere or stick the particles forming thedeveloper of the electrostatic image under the action of a heat flux,for example an infra-red radiation.

Once the development is effected, an image may be observed or noticed onthe paper sheet or web, which image is the true and contrastedreproduction, replica or copy, with the finest details, of the patternor design of the original document. The electroscopie particles with apositive charge have therefore been attracted at the spots or areasWhere the negative charge of the paper sheet or web 1 has not beendischarged in the zone or area 14; it may be noted that in addition thehalf-tints, that is the greys, are truly reproduced; until now, all ofthese qualities could never be obtained to such an extent in xerography.

FIGURE 2 illustrates and explains in a detailed manner, the assumedmechanism of the selective discharge of paper 1 in the area or region14. On this figure may be seen that the photoconductive paper 2 consistsof a paper base 2a and of a photoconductive layer or coat 2b upon whichhave symbolically been shown by small circles, the negative charges Nconstituting the electrostatic image formed on said photoconductivelayer or coat during the optical projection of the pattern or design ofthe original to be copied; these charges N in alternation with theintermediate spaces 211', free or devoid of charges, therefore representthe latent image resulting from said optical projection; these charges Ncorrespond to the blacks of the pattern or design of the original andsaid intermediate spaces 2b correspond to the whites of this pattern ordesign.

It should be noted that the paper base 2a is a very good staticelectricity conductor, whereas the photoconductive layer or coat 2bforms a chain comprising in alternation perfect charged insulators(zones or regions of localisation of the charges N and electricityconductive spaces (intermediate spaces 217); these intermediate spaces2b are therefore connected by a conductive chain to the high frequencycircuit and to the direct current circuit.

The paper sheet or web 1, rolled up round, Wound, curled or wrappedabout the metal drum 4, comprises a paper support, backing or carrier1a, hence a very good static electricity conductor and a thin layer orcoat 1b made of a plastic material which forms an insulating material;this insulating layer or coat has no photoconductive or photosensitivequality; in its upstream portion, having not yet reached the area orzone 14, the insulating layer or coat 1b of this paper sheet or web isnegatively charged as previously stated; the negative charges N of thislayer or coat are symbolically shown by small circles.

Under the influence of the high frequency component of the electricfield, the zone or region 14 is subject to an ionization in conditionssuch that there appears in this zone or area a high frequency plasma,which is a mixture of ions of opposed polarities and of electrons; thecontinuous or DC. component of the field tends to cause the discharge ofthe insulating layer or coat 1b in those portions thereof which areconfronting, opposite to or registering with the intermediate spaces21), free or devoid of charges, of the sheet or web 2; the highfrequency component enables to activate and to regul-arize thisdischarge which corresponds to the removal or departure of the negativecharges N of the insulating layer or coat 1b; the regularity or regularfashion of this discharge is shown by the truth or faithfulness of thereproduction or copy on the paper sheet or web 1 of the electrostaticimage of the sheet or web 2; moreover, this truth or faithfulnesswarrants that there is no discharge opposite to the charges N when theseare located or positioned opposite to the charged insulators formed bythe zones or areas of localization of the charges N on thephotoconductive surface of the sheet or web 2; the activation of thedischarge by the high frequency component consists essentially in thefact that the threshold of the discharge potential or voltage of theinsulating surface of the sheet or web 1 is clearly or distinctly lowerthan the discharge threshold which would be present in the case where nohigh frequency plasma would have been formed or built up in the space 14owing to the use of an electric field that does not comprise any highfrequency component.

The electrostatic image, which is formed in the zone or area 14 on theinsulating layer or coat 1b, is the rightleft inverted or reverse one(that is the figure symmetrical with respect to a plane) of theelectrostatic image present on the photoconductive layer or coat 2b;this inversion or reversal may be automatically compensated during theoptical projection of the pattern or design of the original document, sothat to an original, comprising a noninverted or nonreversed pattern ordesign, an identical noninverted or nonreversed copy or reproduction maybe made to correspond directly; it is conceivable that this compensationalso allows to obtain an inverted or reversed copy or reproduction froman original comprising an inverted or reversed pattern or design; on thecontrary, the absence of compensation produces the right-left invertedor reversed copies or reproductions from originals comprisingnoninverted or nonreversed patterns or designs and conversely; it shouldmoreover be noted that the method according to the invention makesdirectly correspond a positive copy or reproduction to a positiveoriginal and a negative copy or reproduction to a negative original; theterms negative and positive used hereinabove, have the same meaning asin photography or in chemical-photocopying and are not related to thepresence of electric charges.

It should be noted that the action of the high frequency component ofthe field decreases quickly beyond the space 14 corresponding to thezone of closest approach of the drums 3 and 4; beyond this zone orregion, there is no more ionization of the air and no discharge takesplace.

It should be noted that the discharge of the insulating surface iseffected in the conditions mentioned hereinabove which are requisite forthe sharpness and other aforesaid qualities of the reproductions orcopies obtained or achieved by the method of the invention, due to thepresence of the tank or storage of charged particles of differingpolarities, formed by the high frequency plasma in the space 14; it isjust this ion storage tank which leads to the obtainment or achievementof a uniform distribution of all the ionization states which arefavourable to a selective and homogeneous discharge of the negativelycharged parts of the insulating layer or coat 1b lying in front of oroppositely to the conductive intermediate zones 2b of thephotoconductive layer er coat 2b; all these secondary effects, such asLichtenberg figures, trees, stars, etc. encountered with the chargetransfer processes, are removed.

Such transfer processes are for example illustrated by the US. PatentNo. 2,982,647 of May 2, 1961, where is described a transfer effectedfrom a photoconductive surface to an insulating surface; but until now,only poor and not industrializable results have been obtained, onaccount of a large number of secondary ionization effects which make itimpossible to achieve an acceptable copy from an original.

The distribution of negative charges N of the photoconductive surface ofthe sheet or web 2 is not substantially modified or altered afterpassage in the zone 14; therefore, there is no charge transfer from onesurface to the other, but a selective discharge of the chargedinsulating surface of the sheet or web 1, by the inducing effectproduced by the charge distribution over the photoconductive surface ofthe sheet or web 2, owing to the action of said high frequencycomponent.

This absence of modification or alteration of the photoconductivesurface 2 is shown by the two following tests:

The sheet or web 2 is developed and fixed, in a fashion known per se inxerography, after passage in the area or zone 14; an image is obtainedwhich is identical or like that achieved by development and fixation ofthe paper sheet or web 1, but inverted or reversed right-left withrespect to this latter;

The charges deposited on the respective surfaces of both paper sheets orwebs are measured as well as the remanent charges before and aftercopying and it is found or verified that these have not been modified oraltered during the operation of the method.

It is therefore possible to obtain a great number of high quality copiesor reproductions from an initial electrostatic image formed on thephotoconductive layer or coat 212.

It should be noted in addition that the same photoconductive paper 2 mayserve for the obtainment or achievement of reproductions or copies ofnew documents which are different from the preceding ones, provided thatthe first electrostatic image, formed on this paper, be at first effacedby dissipation of the remanent or residual charges after printing off ordeveloping; it sufl'ices therefor to subject the photoconductive paperto the action of an infra-red radiation, and then for a few moments, todarkness, i.e., exclusion of active light.

On FIGURE 3 are seen two electrodes consisting of metal plates betweenwhich are maintained in contact with each other, a photoconductive papersheet or Web 2 and a paper sheet or Web 1 coated with an insulatinglayer, the photoconductive layer 2b of the sheet or web 2 being incontact with or engaging the insulating layer 1b of the paper sheet orweb 1; an electrostatic image has been formed, from an originaldocument, on the photoconductive layer 2b in the manner describedhereinabove; the insulating layer 112 has been uniformly charged withstatic electricity of the same polarity as that of said electrostaticimage; the electrode 42 is grounded and the electrode 43 is connected toa high frequency oscillator 44 on the one hand and to a direct currentsource 45 on the other hand; the detail of the electronic diagram is forinstance that shown on FIGURE 1.

The achievement of the electrical connection between the electrodes 42and 43 enables the simultaneous formation, on the insulating layer 1 bof the sheet or web 1, of all the portions of the electrostatic image ofthe sheet or web 2; the operation is carried out as previously in thedarkness, i.e., with the exclusion of active light; the upper or topelectrode is rendered removable owing to a suitable sliding or pivotingarrangement not shown, a flexible or yieldable spring-like electriccontact 46 allowing such a removable condition; after printing off ordeveloping, the electrical connection between said circuits is broken orinterrupted and the removable electrode 43 is withdrawn or removed so asto space or move the sheet or web 1 away from the sheet or web 2; thedevelopment and fixation of the latent electrostatic image formed on theinsulating layer 1a gives or produces an image which is the true andsuitably contrasted reproduction or copy of the pattern or design of theoriginal document.

The discharge potential or voltage threshold, referred to with respectto FIGURE 2, is practically zero owing to the presence of the highfrequency component; however, for a zero potential or voltage (absenceof the continuous of DC. component), the reproductions or copies areoften too pale, so that in practice, one should contrive to have thepotential of the electrode 43 with respect to the electrode 42 (FIGURE3) or of the drum 4 with respect to the drum 3 (FIGURE 1) be of theorder of at least a few volts, preferably of the order of a few hundredvolts.

It should be noted that in the charge transfer process of the US."Patent No. 2,937,943, reference is made to the application of acritical transfer voltage. The graphs, charts or curves of the drawingsof this patent show for each distance between two conductors, thebreakdown or disruptive voltage of the ambient air. It is interesting tonote here that, due to the fact that the voltage in question is thevoltage of migration of the charges under a critical stress orconstraint of the ambient air, these curves or charts show anextrapolation of the Townsend curve. When there is a breakdown, there isa formation of an ion avalanche due to the multiple collisions with thegas atoms (air), of the electrons accelerated by the field applied tothe terminals of the electrodes.

In the method which is the subject matter of the present invention, thecurves, charts or graphs shown in the American patent referred tohereinabove do not play any part since a method of discharging in a highfrequency plasma is used under conditions such that the discharge beginsgradually when the least induction takes place.

The absence of discharge potential threshold (critical breakdown ordisruptive voltage) may be shown by the fact that the method is stillapplicable while yielding the same results when the continuous componentof the field is produced, in the absence of any direct current source,by the discharge of an electret; on FIGURE 4, which illustrates thisalternative embodiment, an electret 47, consisting of an elastomericsilicone sheet wherein cadmium sulphide has been dispersed, is movertowards or drawn near the electrode 43' of a device besides like that ofFIGURE 3, but comprising no source of direct current such as 45; thedischarge of the insulating surface previously uniformly charged of thepaper sheet or web 1' is operated by progressive or gradual sweeping byparallel rows, since as the high frequency oscillator 44' has been atfirst connected with the electrode 43', one side of the electret 47 isat first applied to this latter electrode which is then graduallycovered by the electret due to a movement of continuous deformation ofthe latter, allowing its discharge by successive parallel rows, whenthese rows are put into contact with said electrode 43.

The device of FIGURE 5 comprises a storage or reserve of photoconductivepaper which is stored in the form of a reel or roll 26 on a mandrel orthe like 27. This paper advances or is fed in the direction of the arrowand passes at first below a charging station 20 (of the scorotron type)the conductor 1% of which is brought by a high voltage source 19a to avoltage of 3,500 to 12,000 volts with respect to the envelope, shell orcasing 190 connected to the ground. The paper 2 thus charged moves pastan exposure station comprising a lens 29 and illumination or lightingmeans 24 and 25 past or in front of which the original .30 moves in adirection opposite to that of motion of the paper 2. After having passedaround the rotary drum 3 where it is put in the presence of a paper 1driven or carried along by the rotary drum 4 under the conditionsdescribed with respect to FIGURE 1, the paper 2 is exposed to a uniformillumination owing to a light source 23 in order to efiace and todissipate the remanent or residual charges of its photoconductive layer,then it is rolled up or wound on and about a mandrel or the like 28. Thecopies or reproductions are made on the paper 1 the insulating surfaceof which has been previously charged by the charging station 22 of thescorotron type (the conductor 21a of which is brought to a high voltagewith respect to the ground by means of a high voltage source 21b). Afterhaving exhausted the paper of the reel or roll 26, for example afterhaving achieved about one hundred copies of the size 21 x 27 cm. whichis usual with papers for ofiice purposes, the reel or roll 33, uponwhich the whole paper 2 is now rolled up or coiled, is substituted forthe reel or roll 26 by permutating or exchanging the mandrels 27 and 28or even the paper 2 is reeled again or rewound in the direction reverseof or opposite to unrolling or unwinding through the medium of the drum3, upon taking care to avoid the formation of reproductions or copies atthe locations of the drums 3 and 4 which could then be spaced or movedaway from each other by an adjusting or control device not shown, duringthis period of re- Iwinding. If the reel or roll 26 initially comprisesa 60 meter storage or reserve of paper 2, about two hundredreproductions or copies of the format 21 x 27 cm. can be effected.

The scorotron type device, shown on FIGURE 6, is the one used in thedevice of FIGURE 5 at 20 or at 22. It comprises a metal envelope, shellor casing 210 in the form of an uncomplete cylinder or barrel having adiameter of the order of 20 cm. and grounded, and an axial conductor21a, consisting of a tungsten wire having a diameter ofmicromillimeters, brought to a potential or voltage of several thousandsof volts by the source 2112; the envelope, shell or casing 21c isextended by a meshed element or member 21d made from insulating materialwhich plays an essential part for the obtainment or achievement of theuniformity of the precharging of the photoconductive paper or of thepaper upon which are effected the reproductions or copies; it is assumedthat this element 21d forms a barrier for the too fast ions movingtowards the surface of said papers, thereby avoiding any overchargethereof; at the same time, this element forms a trap for the possiblefew ions of a sign opposite to that of the ions produced within saidenvelope 21, said first ions being caught or trapped and/or neutralizedby this element; this latter is preferably of low mass per surface areaunit, so as to then form a veil or thin foil which may be advantageouslymade of synthetic material such as polyethylene terephthalate (Mylar) ornylon; ittis assumed that said barrier effected is obtained due to thecharge taken by this element in presence of oxygen ions of the ambientair; the meshes of said veil are preferably three to four in number perlinear millimeter.

The device of FIGURE 7 enables to render the process absolutelycontinuous; it derives directly from that of FIGURE 5, so that theidentical or like elements of both of these figures are designated bythe same reference characters. The support, backing or carrier of thephotoconductive layer is formed here by an endless tape, strip, band orbelt 48. In order to regenerate this photoconductive layer or coat foreffecting a new reproduction or copy, this endless belt 48 passes ormoves at first past or in front of a light source 23 which enables thedissipation of the remanent or residual charges, then in front of orpast the stations 31 and 32 the etfect of which is to restore thesensitiveness of the photoconductive layer with a view to allow quickrecycling or reprocessing of the band, tape or belt. The station 31 is aheat source, preferably a source of infrared radiations and 32 is ascorotron type charging station, modified as described with reference 11to FIGURE 6, the high voltage source of this station being of a polarityopposite to that of the sources 1% and 21b.

In the device of FIGURE 8, the photoconductive layer of the endless belt49 is disposed in parallel relation to the paper 1 over a lengthcorresponding to the spacing of both pairs 5011-5012 and 51a-51b ofrotary cylinders. The electric field with the high frequency componentis provided or built up by means of conductive grids 35 and 36 havingrotary bars loosely mounted on their axes or journals, said grids orgrates being supported by the metal boxes or casings 37 and 38,respectively, wherein an air depression is produced in order to applythe endless belt 49 and paper 1 flat against the grids 35 and 36,respectively. For the sake of clarity of the figure, the showing of thecharging stations for the endless belt and paper as well as the opticalprojection system for the pattern or design of the original to be copiedor reproduced have been omitted. Likewise have been omitted theregenerating stations which treat or process the endless 'belt 49 afterachievement of the copies or reproductions.

The latent images, formed on the paper 1 of FIGURES 7 and 8, aredeveloped and fixed in a manner known per se in xerography.

As to the constitution of the endless tape, band or belt 48 (FIGURE 7)or 49 (FIGURE 8), it should be noted that it is composed of at least twolayers: a first lower or bottom layer made from a suitable materialwhich will impart thereto the physical qualities required for the highnumber of successive processes or operations or unrollings during theprinting off of a large number of copies and a photoconductive upper ortop layer. Said material may be natural or synthetic rubber comprisingsuitable charges or additions which determine its degree of elasticityand/or charges or additions which render it more or less conductive(carbon black issued from the incomplete combustion of kerosene orlamp-oil for example). The rubber used Will be for example a butylrubber or a silicone elastomer. This material may also be a syntheticplastic material or substance such as a butadiene-vinyl chloridecopolymer, a butadiene-styrene copolymer, an acrylonitrile-vinylidenechloride copolymer, a polyvinyl-butyral polyester, the pliolite 3, 4, or6, etc. The photoconductive layer should contain, dispersed in aninsulating synthetic resin, a photoconductive substance such as zincoxide with sensitizing agents for the panchromatic reproduction orcopying, such as for example the orange acridine, the fluoresceine, theeosine, the Bengale pink and the methyleneblue. The upper or top layermay also contain cadmium sulphide as the only or single photoconductivesubstance or as a photoconductive substance associated with said zincoxide, or cadmium sulphide doped with manganese and oxygen to obtain ahigh sensitivity. The photoconductive layer may also contain organicphotoconductive substances such as anthracene, the triazoles, theoxaziles, the thiazoles and the triazines. This layer may have any ownor proper coloration without any inconvenience for the quality of thecopies or reproductions. The paper 1 is preferably a usual or currentpaper weighing about 40 to 70 g./m. and comprising a thin insulatinglayer consisting for example of lithium, calcium or aluminum stearate orpalmitate dispersed in a binder such as methyl polymethacrylate, abutadiene-styrene copolymer, an acrylonitrile-vinylidene chloridecopolymer, butyl rubber, an acrylonitrile-vinylidene chloride copolymer.

It is understood that the present invention should not be construed aslimited to the forms of embodiments described and shown herein whichhave been given by way of examples only, as many modifications, changesand alterations may be effected by those skilled in or conversant withthe art without departing from the scope of protection as defined in theappended claims.

What I claim is:

l. A method of photoelectric copying comprising the steps of forming,from an original to be reproduced, an electrostatic image upon aphotoconductive surface carried by a static electricity conductive base,uniformly charging with static electricity the insulating surface of anonphotosensitive and nonphotoconductive paper sheet comprising aconductive support and an insulating surface, then bringing with theexclusion of active light, said photoconductive surface in the presenceof said insulating surface at a distance thereof comprised between 0.01micron and microns while subjecting the zone wherein both of saidsurfaces are thus in reciprocal contact to the action of an electricfield comprising an alternative current of frequency ranging from 1.2kilohertz to 3000 megahertz and a direct current component, thepotential of which is at least equal to a few volts, and developing andfixing the electrostatic image formed on the insulating surface of saidpaper sheet after having separated the photoconductive surface from saidpaper sheet.

2. A method according to claim 1, wherein said direct current componentis obtained by the induction caused by an electret placed adjacent theconductive support of said paper sheet, on the face thereof opposite tosaid insulating surface.

3. A method according to claim 1, wherein the uniform charge of saidinsulating surface of said paper-sheet is obtained by putting the latterin the presence of a scorotron. V

4-. A method according to claim 3, wherein that part of said scorotronwhich faces said insulating surface consists of a meshed element madefrom insulating material.

5. A method according to claim 1, wherein said continuous direct currentcomponent is obtained by a direct current source connected to saidconductive support of said paper sheet. 1

6. A method according to claim 1, wherein said electrostatic image is areverse right-left image of the original to be reproduced.

'7. A method according to claim 1, wherein said insulating surface isnegatively charged.

8. A method according to claim 1,- wherein said insulating surface ispositively charged.

References Cited UNITED STATES PATENTS 2,879,395 3/1959 VVaikup 250-4952,982,647 5/1961 Carlson et a1. 961

NORMAN G. TORCHIN, Primary Examiner J. C. COOPER III, Assistant ExaminerUS. Cl. X.R.

